Original Research ARTICLE
Climatic controls on C4 grassland distributions during the Neogene: a model-data comparison
- 1Department of Earth Sciences, University of Minnesota Twin Cities, United States
- 2Department of Geography, Florida State University, United States
- 3Department of Animal and Plant Physiology, University of Sheffield, United Kingdom
- 4Department of Biology, University of Washington, United States
- 5School of Geographical Sciences, University of Bristol, United Kingdom
- 6Met Office Hadley Centre (MOHC), United Kingdom
- 7School of Earth and Ocean Sciences, Cardiff University, United Kingdom
- 8College of Forestry, Oregon State University, United States
Grasslands dominated by taxa using the C4 photosynthetic pathway evolved on several continents during the Neogene and Quaternary, long after C4 photosynthesis first evolved among grasses. The histories of these ecosystems are relatively well documented in the geological record from stable carbon isotopes (fossil vertebrate herbivores, paleosols) and the plant microfossil record (pollen, phytoliths). The distinct biogeography and ecophysiology of modern C3 and C4 grasses have led to hypotheses explaining the origins of C4 grasslands in terms of long term changes in the Earth system, such as increased aridity and decreasing atmospheric pCO2. However, proxies for key parameters of these hypotheses (e.g., temperature, precipitation, pCO2) are still in development, not yet widely applied, and/or remain contentious, so testing the hypotheses globally remains difficult. To understand better possible links between changes in the Earth system and the origin of C4 grasslands, we undertook a global scale comparison between observational records of C4 grass abundances in Miocene and Pliocene localities compiled from the literature and three increasingly complex models of C4 dominance and abundance. The literature compilation comprises >2,600 δ13C values of both fossil vertebrates and of paleosol carbonates. We forced the vegetation models with simulated monthly climates from the HadCM3 family of coupled ocean-atmosphere GCMs over a range of pCO2 values for each epoch to model C4 dominance or abundance in grid cells as months per year exceeding the temperature at which net carbon assimilation is greater for C4 than C3 photosynthesis (crossover temperature model); the number of months per year exceeding the crossover temperature and having sufficient precipitation for growth (≥25 cm/yr; Collatz model); and the Sheffield Dynamic Global Vegetation Model (SDGVM), which models multiple plant functional types (C3 and C4 grasses, evergreen and deciduous trees). Model-data agreement is generally statistically weak, suggesting that regional to local ecological interactions, continent specific plant evolutionary histories, and/or regional to local climatic conditions not represented in global scale GCMs may have been equally as strong or stronger in driving the evolution of C4 grasslands as global changes in the Earth system.
Keywords: Miocene, Pliocene, C4 grasses, Carbon Isotopes, Model-data comparison, Vegetation models
Received: 21 May 2018;
Accepted: 04 Sep 2018.
Edited by:Oana Moldovan, Emil Racovita Institute of Speleology, Romania
Reviewed by:David Nelson, University of Maryland Center for Environmental Science (UMCES), United States
Lydie M. Dupont, Zentrum für Marine Umweltwissenschaften, Universität Bremen, Germany
Copyright: © 2018 Fox, Pau, Taylor, Strömberg, Osborne, Bradshaw, Conn, Beerling and Still. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Dr. David L. Fox, University of Minnesota Twin Cities, Department of Earth Sciences, Minneapolis, 55455, MN, United States, email@example.com